This invention relates generally to the field of turbine flow meters, and, more particularly, to a method and apparatus for using volumetric turbine flow meters to measure mass flow through the meter.
It is often desireable to use a mass flow rate meter which provides a weight per time period number of flow through a duct for use in certain applications. The term xe2x80x9cductxe2x80x9d as used in the present application refers to any tube, conduit, pipe, or the like through which a fluid flows.
One major market for such meters is the aircraft industry which wants to have accurate mass flow rates for its aircraft. This allows an aircraft to load the minimum weight of fuel for given flight including an appropriate safety margin. If an aircraft loads more than that minimum weight, it is essentially burning excess fuel to transport that excess weight to its destination. Thus, there is an incentive to provide accurate mass flow numbers for the aircraft industry as well as other applications.
One method is to use two different flow meters to measure mass flow. In one arrangement, a volumetric flow meter insensitive to density is used in combination with a density sensitive meter which is used to determine the density of the fluid. Once the density is known, a relatively simple calculation using the volume flow rate reading from the volumetric flow meter combined with the density yields the mass flow rate. However, such a combination is subject only to limited number of applications over a narrow range of conditions because one or the other flow meter is also sensitive to a number of secondary variables such as temperature, viscosity, and/or Reynolds number.
Turbine meters are often used as the volumetric flow meter, but such meters are very sensitive to the viscosity, temperature and Reynolds number of the fluid being measured. FIG. 1 shows the classical correlation curve of a standard turbine meter. The illustrated curve graphs the meter frequency divided by the fluid kinematic viscosity is a function of the meter frequency divided by the volumetric flow rate. If a turbine meter is operated at varying temperatures thereby varying the viscosity, the flow rate cannot be determined without knowing that viscosity.
It should be noted that when a turbine meter is operating at a given temperature on a specific fluid, it is not necessary to know the viscosity as long as the turbine meter is calibrated at these same conditions. Those skilled in the art will recognize that such conditions are quite rare in the real world.
In most situations, the temperature will vary which, in turn, causes the viscosity to vary as shown in FIG. 4. This variation does require that the meter system be able to determine the viscosity of a given fluid at a given temperature. Often, this is accomplished by using reference tables which plot viscosity versus temperature for a specific fluid. However, the actual viscosity of any given batch of fluid can vary from the viscosity of another batch of the same fluid sufficiently to negate the value of a reference table as is also shown in FIG. 4.
Other volumetric flow meters can be used in a similar fashion but all suffer from the same deficiency as the turbine flow meter. In addition, many other types of flow meters do not have sufficient accuracy to be competitive with the better mass measuring devices.
With regard to density measurements, there are a number of meter used including differential pressure meters such as orifice plates and target meters which are all sensitive to temperature, viscosity and Reynolds number. In addition, such meters are limited as flow sensitivity is a function of the square root of the differential pressure of force signals generated by the meters.
As a result of the problems using a volumetric flow meter and a density meter combination, most current metering systems employ direct mass flow measuring meters for such measurements. One example of such a meter is a Coriolis meter which tend to be quite expensive. However, when direct mass flow measurement is desired, the user has few choices.
Another direct flow meter employs two turbine elements in tandem wherein one turbine element employs straight turbine blades while the second meter uses a more conventional curved design. The two elements are coupled using a torsional spring. As the mass flow rate increases, the torque reaction causes a phase shift between the two blades. The phase shift is a function of the mass flow rate as long as the rotational speed of the two elements is constant. Several designs are used to maintain the constant rotational speed, including a synchronous motor and a centrifugally loaded vane set. These meters, while commonly used in fuel measurements, are not very accurate and are relatively expensive. Thus, there is a need for a more accurate and less expensive method of measuring mass flow.
The present invention meets this need.
It is an object of this invention to provide an improved mass flow metering system which is accurate and inexpensive.
Further objects and advantages of the invention will become apparent as the following description proceeds and the features of novelty which characterize this invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.